[Patent Document 1] JP-A-2003-247371
A proposal has been made in patent document 1, for example, for a natural ventilation apparatus which is comprised of a main arm having one end pivotally secured to an upper portion of a stile of a framed panel and another end pivotally secured to a vertical frame (jamb) of a window frame at a position lower than the one end, as well as a sub arm having one end pivotally secured to an intermediate portion of the stile and another end pivotally secured to a lower portion of the vertical frame, the sub arm being longer than the main arm. The one end of the sub arm and the center of gravity of the framed panel with the window closed are respectively disposed so as to be located on an identical line extending in a vertical direction.